Conventionally, asbestos type friction materials containing asbestos as a main component, have been used as friction materials for making brake pads for cars and the like. However, recently, adverse effects of asbestos on humans have been noticed and so-called asbestos-free friction materials not using asbestos have been developed.
For instance, semi-metallic friction materials using short fibers of steel are practically utilized as asbestos-free friction materials. The semi-metallic friction materials have merits such as good resistance to wearing and to fading. However, known semi-metallic friction materials have disadvantages that they might burn when the braking action generates high temperatures and that short fibers of steel contained in friction materials might cause considerable wear on rotors in contact with the brake pad. Thus, there has been a need to develop non-steel friction materials not containing steel fibers as friction materials free of asbestos.
The non-steel friction materials can solve the problems such as burning and wearing of rotors in contact in the semi-metallic friction materials. However, compared with conventional asbestos type friction materials, such non-steel friction materials are liable to cause brake noise at a frequency lower than 1 KHz and, in some cases, there might be vibrations of a car body at the time of braking, which is undesirable. Thus, the non-steel friction materials involve new problems.